The present invention relates to a dual-frequency antenna which operates in two frequency bands, and more particularly, to a dual-frequency antenna which is suitable for an antenna of a mobile telephone system which makes separate use of two frequency bands.
In general, a plurality of frequency bands are allocated for use in mobile telephone systems. For example, in the PDC system (Personal Digital Cellular telephone system) used in Japan, the 800 MHz band (810 MHz-956 MHz) and the 1.4 GHz band (1429 MHz-1501 MHz) are allocated, whilst in Europe, for example, the 900 MHz band (870 MHz-960 MHz) GSM (Global System for Mobile communications) and the 1.8 GHz band (1710 MHz-1880 MHz) DCS (Digital Cellular System) are used. Two frequency bands are allocated in this manner due to the shortage of usable frequencies that has arisen from the increase in the number of subscribers. For example, in Europe, it is possible to use 900 MHz band GSM system portable telephones throughout the whole of Europe, but within urban regions, it is possible to use 1.8 GHz DCS system portable telephones, in order to supplement the shortage of usable frequencies.
However, a DCS system portable telephone cannot be used in non-urban regions. Against this background, dual-band portable telephones have been developed which can be used in both GSM and DCS systems. These dual-band portable telephones are naturally equipped with a dual-frequency antenna which is capable of operating in the 900 MHz band and the 1.8 GHz band. In general, these dual-frequency antennas are constituted by respective antennas operating at respective frequencies, the two antennas being connected by means of isolating means, such as a choke coil, or the like, in order to prevent either antenna from affecting the operation of the other.
However, if a choke coil is adopted as isolation means, it is difficult to separate the signals across a broad frequency band. In other words, even if a choke coil is provided between antennas operating at respectively different frequencies, if broad frequency bands are used, such as mobile telephone bands, then a problem arises in that the respective antennas are unable to operate independently over the frequency bands, and they each affect the other and prevent satisfactory operation.
Moreover, if a mobile telephone is mounted in a vehicle, then an antenna is installed on the vehicle. A variety of antennas may be used for this antenna, but reception sensitivity can be increased if the antenna is installed on the roof of the vehicle, being the highest position thereof, and hence roof antennas have been preferred conventionally.
However, in a dual-frequency antenna using a choke coil, such as a trap coil, the antenna length will be great, the antenna will project a long way beyond the roof of the vehicle, and hence it will detract from the vehicle design.
The object of the present invention is to provide a low-profile dual-frequency antenna which operates satisfactorily in two different frequency bands, and in order to achieve the aforementioned object, the dual-frequency antenna of the present invention comprises: a linear element section; a crown section provided at the front end of said element section and having a downwardly inclined umbrella-shape; a matching stub for shorting an intermediate portion of said element section to earth; and a folded element which connects the power supply point of said element with the front end of said crown section; in such a manner that the antenna operates in two frequency bands.
In this manner, in the present invention, a folded element is provided connecting the front end of the crown section provided at the front end of the linear element and the power supply point of the linear element. By providing this folded element, it is possible to achieve an antenna operating in two frequency bands, and a frequency ratio of approximately 1:2 is achieved between the two frequency bands at which it operates.
Moreover, since the dual-frequency antenna according to the present invention is provided with a crown section which functions as a top loading element, at the front end of the linear element, it is possible to reduce the height of the dual-frequency antenna. Therefore, the dual-frequency antenna can be accommodated inside a small antenna case, and excellent design can be achieved since the antenna does not project significantly when attached to the roof of a vehicle.
Moreover, in the dual-frequency antenna according to the present invention, it is also possible to bend the front end of the crown section downwards to form a cylindrical section, and to accommodate the antenna inside a case consisting of a metal base having an installing section attachable to a vehicle formed on the lower face thereof, and a cover which fits into the metal base. Furthermore, it is also possible to accommodate a navigation antenna inside the case.